Recently, with the development of the hi-tech electronic industry, miniaturization and lightweight of electronic equipment became feasible, and use of portable electronic devices is increasing. As a power source of portable electronic devices, the need for batteries with high energy density is growing and research for lithium secondary batteries are being actively conducted. A lithium secondary battery includes a cathode, an anode, and an electrolyte solution, and a separator is generally provided between the cathode and the anode to prevent a phenomenon in which a short circuit occurs by a direct contact between the cathode and the anode.
A separator for an electrochemical device has been manufactured from various resins including polyolefin-based polymer resin such as polyethylene or polypropylene. However, polyolefin-based polymer resin such as polyethylene or polypropylene is prone to shrink at high temperature due to having a low melting point of about 130 to 150° C., and it has been noted that a resulting internal short circuit causes a problem of explosion.
Meanwhile, as a method for forming pores in a substrate of a separator for an electrochemical device, a dry pore forming method or a wet pore forming method is well known, and the dry pore forming method melts a material compound, adjusts crystallization kinetics, and forms pores mechanically during stretching, and the wet pore forming method melts a material compound for a separator substrate for an electrochemical device, blends it with a wax and the like, extrudes the blend, and extract the wax to form pores. However, the separator substrate manufactured by this method fails to have a pore tortuosity (straightness) sufficient to allow smooth movement of lithium ions.
Accordingly, there is still the need for development of a separator for an electrochemical device which provides a wide selection of materials and allows for more smooth movement of lithium ions.